This invention relates to a blender for solid particulate material such as plastic pellets. The invention combines the features of those blenders utilizing a central lift or blending column with the features of those blenders using recycle channels (downcomers) in a gravity flow system. The invention is also capable of utilizing a bottom fill technique for utilizing the energy used to supply material to the blending vessel for the purpose of blending the particulate material by means of material recirculation.
Prior to the present invention, material blenders were known which included a vertically oriented vessel with a centrally mounted lift column for recirculating material within the vessel. Typical examples of such blenders are shown for example in U.S. Pat. Nos. 3,276,753; 3,642,178; and 4,194,845.
Also prior to the present material blenders of the gravity type were known. Blenders of this type include a vertically oriented vessel with a plurality of downcomers each having inlets at various levels in the vessel. Material in the upper part of the vessel enters the downcomers into a receiving bin or hopper so that material from various levels in the vessel are mixed. In some instances a material recirculation system is provided Typical examples of such blenders are shown for example in U.S. Pat. Nos. 3,158,362; 3,216,629; 3,421,739 and 4,068,828.
Prior to the present invention it was also known to utilize a bottom fill technique with a central lift column for blending solid particulate material such as plastic pellets. Such apparatus are generally shown in U.S. Pat. No. 4,569,596 and U.S. patent application Ser. No. 680,213 filed Dec. 10, 1984, now U.S. Pat. No. 4,573,800 both assigned to the assignee of the present invention. In this type of system, the material to be blended is pneumatically conveyed from a source of material to the bottom of the blender and the energy utilized for conveying the material to the blender is used to lift the material up the central lift column entraining material already in the vessel lifting the same to the top of the vessel and thereby blending the material.
In order to improve the supply of material from the top of the vessel to the lift column attempts have been made to utilize recirculation channels or internal downcomers in combination with a central lift or blending column. One such arrangement is shown in U.S. Pat. No. 3,386,707 issued June 4, 1968. When an attempt was made to combine the central lift column with peripherial downcomers or recycle channels and bottom filling of the vessel it is discovered that there is no adequate improvement in the recirculation of material from the top of the vessel down to the lift column to join the fresh material being supplied to the blender. The gaseous fluid under pressure used to lift material up through the blending or lift column tends to be diverted from being directed up the lift column to fluidize material in the vessel thereby bypassing the lift column and interfering with material recirculation.
U.S. Pat. No. 3,386,707 attempted to solve this problem by utilizing a belled lift pipe, but in this apparatus the recirculating air supplied to the lift pipe is supplied through either a vacuum or through a conduit which extends into the lift pipe. Neither of these arrangement is suitable for a bottom filled vessel.
In blenders utilized for blending solid particulate material such as plastic pellets which utilize a gravity type apparatus for recirculating material, it is known that if a vertical recycle channel or downcomer is placed in the blending vessel and that recycle channel includes a plurality of vertically spaced apart openings along its length, material will normally only flow into the channel from the uppermost opening which is buried in the vessel. Thus, despite the several openings in the channel, material only flows into the uppermost opening down through the channel to the channel's outlet. Once the level of material in the vessel falls below an opening in the recirculating channel, then material will start flowing into the channel in the next lower opening.
It is known from U.S. Pat. Nos. 4,560,285 issued Dec. 24, 1985; 4,068,828 issued Jan. 17, 1978 and 3,216,629, issued Nov. 9, 1965, that if a baffle is placed in the channel opening then there will be flow of material into the channel not only from the uppermost channel inlet, but also a lower channel inlet which includes the baffle. As used in a gravity type blender, this creates the ability to withdraw material from not only the uppermost point in the vessel, but also from a lower point to thereby improve blending efficiency.
With prior practice such as illustrated in the above referenced patents, recirculation of the material is typically through an external means whereby material is withdrawn through the channels, supplied to a pneumatic conveying system, recirculated outside the vessel to the top of the vessel for further blending. In addition, in such apparatus, once the position of the blender openings is set by location of the baffle arrangements, no suitable arrangement is provided for controlling the withdrawal of material from various levels in the blender. While U.S. Pat. No. 4,560,285 considers an adjustable baffle, it would be useful to provide some means external to the vessel for adjusting the level in the blender from which material flows into the recycle channels so that in some instances material may be supplied to the recycle channel from the uppermost opening and the next lower opening or from the uppermost opening and an opening further down the recycle channel. It would also be useful to provide some means for controlling flow of material through the downcomers.